This invention relates to printers that print characters on sheets of a printing medium and, more particularly, to a mechanism that deposits each completed sheet onto a collection stack.
Printers are devices that print characters onto a printing medium such as a sheet of paper, and then deposit the sheets onto a stack for later pickup by the user of the printer. Printers are of many types, and are commonly linked to a computer that supplies the text or figures that are to be printed. The present invention relates to sheet printers, which print on single sheets of paper, as distinct from printers that print onto a continuous printing medium that may later be separated into individual sheets.
Many printers use a pigment-containing liquid, generally termed an ink, to form the characters on the printing medium. (By contrast, other printers use a dry toner to form the characters.) The printer delivers the ink to the medium in the proper pattern using a printhead that creates the proper patterning. One important type of printhead is the ink jet printhead, which forms small streams or droplets of ink that are ejected toward the printing medium in the pattern that forms the characters. The droplets strike the medium and then dry to form the permanently printed characters. The drying occurs by a combination of absorption into the printing medium and evaporation of the liquid component of the ink.
The drying of the droplets takes some time, depending upon the content and composition of the ink, the weather (increased humidity slows the drying process), and the character of the printing medium. Different types of paper absorb the ink and permit it to dry at different rates. Specially treated papers that cause rapid drying of the ink are available for ink jet and other types of printers that use liquid inks, and such types of papers are recommended for use with such printers. One type of absorbent paper utilizes a thin, highly absorbent clay coating. Alternatively, other methods such as heaters or air jets can be used to accelerate the drying of the ink, but such methods increase the cost, weight, and complexity of the printer.
Many users of liquid-ink printers utilize conventional paper, not treated to accelerate drying, in their printers. The deposited ink dries much more slowly on the conventional paper than when the proper treated paper is used. As a result, there is an increased tendency for the ink to smear on the top sheet of the stack as the next sheet is conveyed onto the stack, and the bottom of the next sheet slides over the top of the top sheet. The user of the printer is likely to be unsatisfied with the results, and may blame the printer for the smeared characters rather than recognize that the use of the incorrect paper is responsible. This problem becomes particularly acute as printers are made to operate at increasingly high speeds, reducing the time available for the ink to dry on a sheet before the next sheet slides across it.
There therefore exists a need for an approach to reduce the smearing of ink on the previously deposited top sheet of a stack, as the next sheet is completed and deposited thereupon. The present invention fulfills this need, and further provides related advantages.